Shortly after Jasinksi’s report it was discovered that a heat treatment step under inert atmosphere increases the stability and activity of metal macrocycles. [92] Fundamental work of Jahnke, Bagotsky, Wiesener and Fuhrmann and later van Veen established ac- tivity trends, focusing on the pyrolysis of transition metal porphyrins and phtalocyanins adsorbed onto carbon blacks. [92, 94, 95, 106–111] The heat treatment was conducted
under an inert atmosphere of N2, He or Ar or under vacuum at temperatures between
300°C and 1000 °C. An optimum temperature range of 600°C to 800 °C was established for these precursors. [96, 106]
Figure 1.17: Increase in (a) ORR activity and (b) stability of different metal-
macrocycles adsorbed onto Norit carbon black after heat treatment [Taken with per- mission from Ref. [107]]
Figure 1.17 (a) shows the general trend of increasing the activity of differently substituted metallo-porphyrins and -phthalocyanins adsorbed onto carbon black upon heat treatment. It can also be seen in Figure 1.17 (b) that a higher temperature yields a material with higher stability. [96] It was established that the order of activity for the 3d metal centres is: Fe > Co > Ni ∼= Mn. [96] Although the activity could be increased, it was still insufficient to be considered a viable alternative to Pt in fuel cells. An additional major problem was the complex synthesis of these macrocycles, offsetting the price benefit over Pt due to the low cost of the precursors. [112] It is proposed that the macrocycle might retain its nitrogen coordination to the metal centre and therefore the activity stems from the metal center as it does for the non-heat treatet material. [113–115] Gupta et al. reported the first synthesis of heat treated M-N/C catalysts with a non-macro-cyclic precursor, utilising a simple metal salt of Fe(II) or Co(II) and poly-acrylonitrile on carbon black. [116] This showed that a macrocycle was not necessary in order to form highly active materials upon
heat treatment and opened the way for the development of heat treated M-N/C catalysts prepared with a wide range of different nitrogen and metal precursors. [96, 117] Therefore the synthesis of ORR catalysts at a significantly lower cost as compared to the macrocycles is possible. Further research established that virtually any mixture of nitrogen precursor and metal salt will yield a material with some ORR activity. [96, 117] However, the final activity and stability strongly depends on the choice of precursors and synthesis conditions, e.g. temperature, duration, heating rate and gas atmosphere. [117] Generally the synthesis of highly active M-N/C materials is divided into 3 major groups [96, 117]: (1) catalysts from precursors based on transition metal macrocycles; (2) catalysts derived from high surface area carbon in the presence of a metal salt and a gaseous nitrogen precursor, e.g. NH3; (3) catalysts prepared from a metal salt and nitrogen containing
molecules, either in the presence of carbon black or a leachable template, such as SiO2
nanoparticles. Although an N4-containing precursor is not crucial to obtain highly active
catalysts, a nitrogen source is. [96, 117] A wide variety of nitrogen precursors have been used to prepare NPMCs. [96, 117]
Figure 1.18: General structure of different precursors used for the incorporation of ni-
trogen groups into M-N/C catalysts (top) small N-containing molecules (a) acetonitrile (b) cyanamide (c) aliphatic terminal diamines (middle) aromatic N-containing com-
pounds d) perylene-tetracarboxylic dianhydride in combination with gaseous NH3 e)
1,10-phenantroline (bottom) polymeric precursors f) polynitroaniline g) polyaniline
Figure 1.18 shows some typical examples of nitrogen containing molecules used as nitro- gen source to prepare M-N/C catalysts. [96, 117] They can be subdivided into i) small molecules ((a) - (c)) ii) aromatic compounds ((d) and (e)) and iii) polymerised compounds ((f) and (g)). [96, 117] The right combination of precursors and heat treatment regimen can lead to a high surface area material with high conductivity, chemical resistance and high ORR activity. [96, 117] Most publication focus on Fe and Co as the metal source,
as these have shown to yield the most active materials. [96, 117] Although early appli- cations in working PEMFC single cells showed a low activity as compared to Pt [118], successive optimisation resulted in materials whith promising potential. [62, 117, 119–121] Notably the work of the Dodelet and the Los Alamos National Laboratory (LANL) groups pioneered the development of these materials. [62, 117–121] Examples of highly active cat- alysts with promising performance in PEMFCs were prepared by i) the polymerisation of aniline over carbon black, in the presence of a mixture of an Fe and/or Co salt and successive heat treatment at 900°C under Ar (LANL) [120] ii) the ball milling of carbon black with a mixture of 1,10-phenanotroline and iron-acetate, heat treated under an at- mosphere of Ar and/or NH3 (Dodelet) and lately [62] iii) the pyrolysis of iron-acetate and
1,10-phenantroline and a microporous metal organic framework as template, e.g. ZIF-8 (Dodelet). [119] Usually the catalysts are refluxed in a solution of acid after heat treat- ment, e.g. 0.5M H2SO4, in order to remove unreacted metal residues. A second heat
treatment after this acid leaching step has shown to improve the activity. [117]
Figure 1.19: (a) Polarization curves for MEAs comprising a cathode made with Fe-
ZIF8-derived catalyst (blue stars) and Fe-N/C from Lefvre et al. [62] (red circles). For comparison, the polarization curve of an MEA made with a state-of-the-art Pt-based
cathode with a loading of 0.3 mgPt cm2 (green squares). (b) Power density curves cor-
responding to polarization curves shown in a. For the two MEAs made with iron-based
cathodes, the catalyst loading was 3.9 mg cm2. [Taken and adapted with permission
from Ref. [119]]
Figure 1.19 shows the single fuel cell performance, comparing a state of the art Pt/C catalyst to a newly developed Fe-N/C catalyst which was prepared via the decomposition of iron-acetate and 1,10-phenantroline over ZIF-8 and to an older Fe-N/C variant of the Dodelet group, which was prepared with the same metal and nitrogen source, but with carbon black as a support. [119] The increased activity is attributed to the higher surface area which then leads to a larger amount of active sites accessible. [119] Although the performance looks promising, the loading of the M-N/C catalyst in this study is 10 times higher than typically used for Pt (4 mg cm-2 versus 0.4 mg cm-2). The argument
is the negligible price, which in theory allows a much higher loading. [38] However, as discussed in 1.5, considerations of electrode volume need to be taken into account, as mass transport effects will be detrimental at a certain electrode thickness, especially as lower concentrations of oxygen are used, such as the 23% present in air. The use of a significantly higher loading in M-N/C material results in a significantly higher thickness as compared to Pt based materials of 100μm to 10 μm, respectively. The implementation of this material in single cells and associated challenges will be discussed in Chapter 8. To establish structure-property relationships on this type of material is challenging, as the parameters that influence the activity are multidimensional and varying a parameter such as the precursor ratio might change several properties such as surface area, degree of graphitization, accessible surface active sites etc. [117] Some general trends that emerged from studies vary certain properies while trying to keep other conditions constant are as follows [117]:
• the activity increases with an increase in heat treatment temperature for the same precursor up to 900 - 1000°C and decreases thereafter. [120]
• the peroxide yield decreases with a higher BET surface area. [120]
• a transition metal ion, even in trace amounts increases the activity significantly. [122, 123]
• increasing the metal content in the precursors increases the activity to a certain level of∼0.5 wt%, levelling at higher concentrations and then dropping at very high concentrations∼10 wt%. [114, 124]
• a higher proportion of micropores in the same material leads to a higher activ- ity. [125]
• a higher BET surface area in the same material leads to a higher activity. [117] • acid leaching and subsequent second heat treatment increases the activity and sta-
bility. [117]
• there is a trade-off between stability and high activity, as a second heat treatment step in NH3 increases the activity. However, this increased activity has a lower
• there seems to be a correlation with the content of certain nitrogen functionalities, such as pyridinic and quarternary, and the catalytic activity. [126]
• the activity increases with the duration, the material is held at the optimal tem- perature until it levels off after around 40 - 60 min. This is presumably when the activity forming reactions is completed. [127]
• Fe seems to produce the most active catalysts among the 3d metals. [45, 117]